1. Field of the Invention
This invention relates to materials and methods of manufacturing spark plug electrode tips which provide substantially improved erosion resistance and enhanced durability of spark plugs in both leaded and unleaded fuel environments for internal combustion engines.
2. Brief Description of the Prior Art
Spark plugs are used in internal combustion engines to ignite fuel in a combustion chamber. The electrodes of a spark plug are subject to intense heat and an extremely corrosive atmosphere generated by the formation of a spark and combustion of the air/fuel mixture. To improve durability and erosion resistance, the spark plug electrode tips must be able to withstand the high temperature and corrosive environment of the internal combustion chamber resulting from the chemical reaction products between air, fuel and fuel additives.
SAEJ312 describes the specification for automotive gasoline used as a fuel in the United States. The gasoline consists of blends of hydrocarbons derived from petroleum: saturates (50-80%), olefins (0-15%), and aromatics (15-40%). Leaded gasoline contains about 0.10 g Pb/gallon fuel (0.026 g Pb/L), and 0.15% sulfur. In unleaded gasoline there is about 0.05 g Pb/gallon, (0.013 g Pb/L), 0.1% sulfur, 0.005 g P/gallon, (0.0013 g P/L). In addition, there are a number of additives incorporated into the fuel for various reasons. For example, tetramethyllead (TML) and tetraethyllead (TEL) are added as antiknock agents. Carboxylic acids (acetic acid), compounds are added as lead extenders. Aromatic amines, phenols are added as antioxidants. Organic bromine, chlorine compounds are added as scavengers and deposit modifiers. Phosphors and boron containing compounds are added to reduce surface ignition, preignition and as engine scavengers. Metal deactivators are added to reduce oxidative deterioration of the fuel by metals, such as Cu, Co, V, Mn, Fe, Cr and Pb. In addition, carboxylic acids, alcohols, amines, sulfonates, phosphoric acid salts of amines, are used as rust-preventing additives.
The mechanism for ignition in an internal combustion engine is very complex and is briefly discussed here. In the gasoline engine, the rising piston compresses the fuel/air mixture, causing increases in pressure and temperature. The spark ignites the fuel-air charge, and the force of the advancing flame front acts against the piston, compressing the unburned fuel-air charge further. Preflame combustion reactions occur in the unburned fuel-air mixture. The pinging noise or knock often associated with internal combustion engines is produced when an extremely rapid combustion reaction occurs in the end gas ahead of the advancing flame front. The formation of the preflame reaction products of the gasoline sets the stage for knock. It is believed that the alkyllead additive must first decompose in the combustion chamber to form lead oxide before it can exert its antiknock effect. The antiknock species must be finely dispersed in the combustion chamber so that adequate numbers of collisions of the critical reacting species with the antiknock agent will occur. However, lead oxide deposits can cause problems of valve burning and spark plug fouling. Lead deposits which accumulate on the spark plug insulator cause engine misfiring at high speed due to the relatively high electrical conductivity of the deposit.
The complete combustion of a hydrocarbon fuel with air will produce carbon dioxide (CO.sub.2), water (H.sub.2 O) and nitrogen (N.sub.2). The ratio of air to fuel by weight, 14.5/1, is the chemically correct mixture ratio. When less air is available, some carbon monoxide (CO) and hydrogen (H.sub.2) are found in the products, whereas if excessive air is available some oxygen (O.sub.2) is found in the products. The atmosphere present during the combustion may cause the hot corrosion of electrodes in the spark plug.
The manufacture of copper (Cu) and nickel (Ni) electrodes for spark plugs is a proven art and has been accomplished in a variety of ways. For instance, U.S. Pat. No. 3,803,892 issued Apr. 16, 1974 and entitled "Method of Producing Spark Plug Center Electrode" describes a method of extruding copper and nickel electrodes from a flat plate of the two materials. U.S. Pat. No. 3,548,472 issued Dec. 22, 1970 and entitled "Ignition Plug and Method for Manufacturing a Center Electrode for the Same" illustrates a method of cold forming an outer nickel cup shaped sleeve by several steps, inserting a piece of copper wire into the cup and then lightly pressing the two materials together. U.S. Pat. No. 3,857,145 issued Dec. 31, 1974 and entitled "Method of Producing Spark Plug Center Electrode" discloses a process whereby a copper center core is inserted into a nickel member and attached thereto by a collar portion to assure that an electrical flow path is produced.
The spark plug electrodes produced by the methods disclosed above perform in a satisfactory manner for a relatively short period of driving time when used in vehicles that were manufactured prior to the implementation of the clean air act of 1977 in the United States. After 1977, with modifications to engine and fuel, the operating temperature of most vehicle increased. As a result of the changes in the engines and fuels, some of the operating components in engines have been subjected to the corrosive effects of the exhaust gases. After a period of time of operating at higher temperatures in recirculation gases, some corrosion/erosion can occur at the nickel-based center electrode. Once corrosion has taken place, the electrical flow path deteriorates which can result in lower fuel efficiency.
U.S. Pat. No. 4,705,486 issued Nov. 10, 1987, and entitled "Method for Manufacturing a Center Electrode for a Spark Plug," discusses a method for manufacturing a center electrode in order to provide some degree of longevity of the spark plug. The center electrode is made from a good heat conducting material such as copper surrounded by a jacket of a corrosion resistant material such as nickel.
This invention also provides a method of manufacturing an electrode for a spark plug whereby a platinum tip is attached to a body composed of a nickel alloy such as Inconel (a registered trademark of International Nickel Company) nickel alloy (e.g., Inconel) body in which a copper core is located. In this process, a blank is cut from a roll of Inconel wire and the end face squared to produce a flat surface. A strip of platinum is welded to the flat surface and a chamfer surface is produced on the first end to remove any flash remaining from the weld and produce a platinum tip. The blank is placed in a die and extruded to produce a cylindrical bore. As the extrusion takes place, the platinum flows down the chamfer to completely cover the weld. A copper core is inserted in the cylindrical core such that there is an electrical flow path produced between the platinum tip and copper core through the Inconel body. The platinum tip is resistant to the corrosive component in a combustion chamber and maintains an electrical flow path between a copper core and a ground. The platinum is extruded to cover the weld to assure that the flow path does not deteriorate under normal operating temperatures.
Platinum as an electrode tip base alloy is a preferred candidate material due to the fact that it resists oxidation and has been proven to greatly increase the service life of a spark plug. This design, including a platinum tip has increased the service life of spark plugs at least a factor of two over standard spark plugs, not incorporating a platinum electrode tip. However, the platinum tip must be used in the unleaded fuel environment. In the leaded fuel environment, lead oxide may react upon heating with H.sub.2, B, K, CaC, Na, C and CO and reduce to metallic lead. Metallic lead may attack platinum tip to form a low melting eutectic. The local melting of platinum will reduce the durability of spark plug. Platinum is also a very expensive metal which significantly impacts the final cost of a spark plug. Currently, platinum is selling in the range of $380 US dollars per troy ounce.
The life of a spark plug is improved by joining a heat-resistant and wear-resistant layer of platinum to the spark-discharge end of the center electrode which is made of Inconel. Resistance welding is used to minimize wear of the spark discharge end of the center electrode. However, the joint between the platinum tip and center electrode base metal often becomes cracked and oxidized. In certain cases, the platinum tip falls off at the position of this cracked and oxidized interfacial boundary. This phenomenon is considered to be largely due to thermal fatigue stress caused by the difference in linear expansion coefficients between platinum and Inconel. As a result of the varying operating loads of the engine, the spark plug is alternately and repeatedly subjected to high and low temperatures. Owing to this alternate repetition of high and low temperatures and to the difference in linear expansion coefficients, the joint between the discharging layer and the center electrode is repeatedly subjected to the thermal stress which results in the formation of transverse cracking.
U.S. Pat. No. 4,540,910 issued on Sep. 10, 1985, entitled "Spark Plug for Internal-Combustion Engine," recognizes this shortcoming and discloses a method of using a thermal stress relieving layer disposed between the discharging layer and the base metal of the center electrode. The thermal stress relieving layer is made of a platinum base alloy containing nickel, which is also present in the base metal. The discharging layer may be made of a material consisting essentially of 70 to 90 wt. % platinum and 30 to 10 wt. % iridium. The thermal stress relieving layer may be made of a material consisting essentially of 5 to 95 wt. % platinum and 95 to 5 wt. % nickel. Another platinum-containing wear-resistant layer may be provided on the other electrode. The wear-resistant layer is made of a material consisting essentially of 5 to 60 wt. % nickel and 95 to 40 wt. % platinum. However, at high temperatures under oxidizing atmosphere, the selective oxidation of nickel in platinum-nickel alloys may occur which will produce a porous oxide layer. This oxide layer may be removed by spark discharge. The combination of selective oxidation and material removal by spark erosion will significantly reduce the life of a spark plug.
There is still a need to develop a long-life non-platinum alloy and manufacturing method for spark plugs electrodes used in internal combustion engines in leaded and unleaded fuel environments.